Douglas Carswell’s victory in Clacton is, of course, a headline-grabber. But it was the UKIP surge two-hundred miles further north that was the real shock.

In the past, people like Lord Ashdown used to dream of a historic realignment of the centre-left, pointing out the combined votes for Labour and the Liberal Democrats usually accounted for more than half the total.

Labour’s new MP, Liz McInnes, scraped a 600 vote majority, a knife-edge result no poll had predicted, and was not unreasonably ridiculed for calling her win “backing for Ed Miliband’s plans”.

Interviewed afterwards, she appeared unable to respond to questions about the UKIP surge in a Labour heartland, falling back instead on the kind of rehearsed soundbite that plays into the hands of complaints about an out-of-touch Westminster elite.

The sudden realisation that UKIP is soaking up Labour as well as Conservative votes seems to have left the party like a rabbit in the headlights.

They’d assumed UKIP was nothing but good news for them, stealing Tory votes and making it possible for Labour to win on not much more than one-third of the popular vote. Suddenly that looks a very dubious strategy.

Polling suggests UKIP are picking up support in the north-east, parts of the north-west and west Midlands, and even in Liberal strongholds in Cornwall.

The assumption was always that the UKIP vote would fizzle out as the general election approached, and it’s still hard to believe they’ll achieve the mid-teens they’re currently getting in polls.

Nigel Farage talks about holding the balance of power after next May. That still seems hopelessly optimistic -- but there’s no doubt he’s turned UKIP into a broad “anti-politics” coalition, reaching beyond disgruntled Conservatives to people who would never dream of voting Tory.